1) Field of the Invention
The present invention relates to an exhaust gas cleaning device constructed of a metal-made honeycomb core body for carrying an exhaust gas cleaning catalyst. As a cleaning means for exhaust gas from a motor vehicle, the exhaust gas cleaning device is generally installed at an intermediate point of an exhaust pipe.
More specifically, this invention is concerned with an exhaust gas cleaning device having a special structure suitable where exhaust gas contains a lot of HC (hydrocarbons) and exothermic reactions abruptly take place between the exhaust gas and an exhaust gas cleaning catalyst or where a preheater is required to meet temperature conditions suitable for a catalyst layer upon activation of an exhaust gas cleaning device of the above sort.
2) Description of the Related Art
Exhaust gas cleaning devices of the above sort have conventionally been fabricated in the following manner. A planar band made of a heat-resistant steel sheet and a corrugated band obtained by forming a similar steel sheet into a wavy or sinuous form are superposed one over the other in a contiguous relationship and are rolled together into a multilayered spiral form, thereby forming a honeycomb-shaped multilayered composite body (hereinafter called the "honeycomb core body"), axially defining a number of network-patterned gas flow passages for allowing exhaust gas to flow therethrough. The honeycomb core body is then inserted into a cylindrical metal casing which is open at both ends thereof.
The honeycomb core body and metal casing are thereafter firmly joined together by brazing or welding, so that they can withstand thermal expansion and stress due to the high temperature of exhaust gas, exothermic reactions of the exhaust gas induced by the catalyst, and the like and further, vibrations while the associated motor vehicle is running. Needless to say, the planar and corrugated bands which make up the honeycomb core body can be firmly joined together at contacts therebetween by any one of various suitable methods.
There is however a recent move toward constructing an exhaust gas cleaning device without a metal casing which is adapted to receive and firmly fix a metal-made honeycomb core body. In other words, with the metal-made honeycomb core body alone, i.e., without the need to provide a metal casing fixed to the outside of the metal honeycomb body, the structure of this invention has an advantage from the standpoint of the price competition with, for example conventional cordierite ceramic carriers. The omission of such an outer casing naturally leads to a reduction in the cost for inserting a metal-made honeycomb core body in the metal casing and then uniting them together, i.e., in the so-called canning cost, whereby substantial cost savings can be realized.
The conventional exhaust gas cleaning devices constructed of a honeycomb core body alone and the conventional exhaust gas cleaning devices constructed of a honeycomb core body and a metal casing are however both unsuitable, for example, for the following application purposes because of the structure of the honeycomb core body.
The conventional honeycomb core body requires further improvements, for example, for such application purposes as to be described next since the planar band and the corrugated band are rolled tight from the center into the multilayered form.
(i) Upon cleaning exhaust gas of a high HC concentration as is observed in the case of a motor cycle or the like, the flow rate distribution of the exhaust gas is highest at a central part (i.e., the center of rolling) of the honeycomb core body. This central part is therefore exposed to abnormally high temperatures due to catalytic exothermic reactions of the exhaust gas induced by an exhaust gas cleaning catalyst such as Pt or Pd, whereby thermal stress is developed and the durability of the device itself is lowered accordingly. As is known well, the temperature inside an exhaust gas cleaning device of this sort generally ranges from 700.degree. C. to 800.degree. C. This temperature may, however, increase to as high as 1,200.degree. C. where HC is discharged at a high concentration.
(ii) When it is desired to activate a honeycomb core body of this sort, the temperature of exhaust gas has not yet risen to a temperature level optimal for catalytic reactions in many instances. A heating means can be used to heat up the catalyst zone to the optimal temperature. In some cases, an additional exhaust gas cleaning device is therefore used as a preheater so as to effectively use heat to be produced by catalytic exothermic reactions of exhaust gas as has been described above. Use of the above-described conventional honeycomb core body however leads to a substantial pressure loss, resulting in a reduction in the engine output.
If the openings of the network-patterned air flow passages are enlarged or coarsened to reduce the pressure loss (resistance to flow) due to the honeycomb core body, the rigidity of the honeycomb core body becomes insufficient so that the durability of the honeycomb core body is reduced.